Okay, here’s a way out there theory: Maybe Tristan’s sister didn’t die in the training exercise. Maybe she was horribly injured and ended up in a red Darth Vader suit, and now calls herself Mariah. If she blamed the government for her accident, it would explain her turn to the dark side. Of course, this theory is based entirely on the rules of thumb that no-one is mentioned unless they are part of the story, and important characters are much more likely to be related to other important characters than in the real world.